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easyBasicsSometimes asked

Check Strings Equal

Basics · 6 min

Given two strings a and b, return whether they are exactly equal (same length, same characters in order).

In plain English

Different lengths → not equal. Otherwise compare character by character.

Examples

Example 1

Input: a = "abc", b = "abc"
Output: true
Note: Identical.

Example 2

Input: a = "abc", b = "abd"
Output: false
Note: Last char differs.

Example 3

Input: a = "ab", b = "abc"
Output: false
Note: Different lengths.

Constraints

0 ≤ a.length, b.length ≤ 10^4

Clarifying questions1First question: Case-sensitive?Explore questions

Q1.Case-sensitive?
Assumed yes; equalsIgnoreCase for case-insensitive.

Quick interview revision

Understand

30-second explanation

Build the interview-ready mental model before comparing implementations.

If lengths differ, return false. Else compare each index; the first mismatch returns false. In Java use `.equals`, never `==` (which compares references).

Interview contextWhat is testedWhere candidates slipHow to stand outExplore interview context

What's being tested

Whether you know Java's == vs .equals distinction and compare correctly.

Where candidates slip

Comparing String references with ==.

How to stand out

Contrast Java (== is reference) with Python (== is value).

Choose

Compare the available approaches

Use constraints and trade-offs to select the route you can defend in an interview.

Learn & trace

Build each solution step by step

Move from the core idea to a visual trace, implementation, proof, and edge cases.

03.1

Approach 1 of 1 · Optimal

Character comparison

TimeO(n)SpaceO(1)

This is the standard, expected solution — lead with it.

Stuck? Progressive hintsReveal one at a time

1Hint 1Explore hint

What two conditions make strings equal?

2Hint 2Explore hint

Check length, then compare characters in order.

3Hint 3Explore hint

In Java, why is `a == b` the wrong way to compare strings?

A · Core idea

Two strings are equal when they have the same length and the same characters in order. Check length first, then compare character by character, returning false on the first mismatch.

The Java `==` trap. In Java, == on String objects compares references (are they the same object?), not contents — so a == b can be false for two equal strings. Always use .equals() for content equality. This is one of the most common fresher bugs.

Why length-check first. Different lengths can't be equal, and the check lets the loop index both strings safely with one bound.

Python. a == b does compare contents, so the language difference itself is worth calling out.

B · Visual dry run

Interactive dry run

Select a frame to connect the decision with its visual state.

a = "abc", b = "abd"

len

3 == 3

Stateok

Final resultfalse

C · Code with teaching comments

solution.javaJava

Character comparison — optimal

2 teaching notes

1
public boolean equal(String a, String b) {
2
    // Different lengths can't be equal.
3
    if (a.length() != b.length()) return false;
4
    for (int i = 0; i < a.length(); i++) {
5
        // First differing character.
6
        if (a.charAt(i) != b.charAt(i)) return false;
7
    }
8
    return true;
9
}

Yellow row = the algorithm's core logic. Amber `//` lines above it = what you'd say out loud.9 lines

D · Why it works and complexityO(n) timeO(1) spaceOpen the invariant and proofExplore the proof

The == vs .equals distinction in Java is one of the most common fresher bugs — call it out.

ComplexityO(n) time, O(1) space — length check is O(1), then up to n character comparisons with early exit.

E · Edge cases and pitfalls5a = "abc", b = "abc" — Identical → true.Explore edge cases

Edge cases handled

a = "abc", b = "abc"Identical → true.
a = "ab", b = "abc"Length differs → false.
a = "", b = ""Both empty → true.

Pitfalls

Using `==` on Strings in Java (compares references, not contents).
Skipping the length check.

Practice

Try the solution yourself

Edit the optimal implementation, test another input, and observe the result.

Editable code previewpublic boolean equal(String a, String b) { if (a.length() != b.length()) return false; for (int i = 0; i < a.length(); i++) { if (a.charAt(i) != b.charAt(i)) return false; }Explore the playground

Try it — Java· run a complete program; method snippets need a driver

Avoid mistakes

Recognize the interview traps

Understand why common attempts fail before reviewing the corrected implementation.

Using == on Java Strings

== compares object identity; two distinct String objects with the same text are not ==. Use .equals for contents.

Explore the code comparison

Don't write this

if (a == b) return true;     // reference comparison

Write this instead

return a.equals(b);          // content comparison

Revise

Carry the pattern into the interview

Quick interview revision

Pattern to remember

Length + char comparison

Code pattern to memorize

len equal AND every a[i]==b[i]   (Java: use .equals)

Rules to remember

1Length must match.
2Compare chars in order.
3Java: use .equals, not ==.

Use this pattern when

Content comparison of strings

Choose another pattern when

Java == on strings — a reference-comparison bug

Interview takeaway

In Java, == on strings is a reference bug — use .equals.

Continue

Transfer the pattern to another problem

Use a direct variation or pattern sibling to prove the idea is reusable.

Transferable patternA core fresher pattern: Length + char comparison. Master it on small inputs and it transfers to harder problems.

Practice queueDirect variations and pattern siblings

Direct follow-ups

Valid Anagram
Equality of letter multisets, not order.

easyBasicsSometimes asked

Check Strings Equal

Basics · 6 min

Given two strings a and b, return whether they are exactly equal (same length, same characters in order).

In plain English

Different lengths → not equal. Otherwise compare character by character.

Examples

Example 1

Input: a = "abc", b = "abc"
Output: true
Note: Identical.

Example 2

Input: a = "abc", b = "abd"
Output: false
Note: Last char differs.

Example 3

Input: a = "ab", b = "abc"
Output: false
Note: Different lengths.

Constraints

0 ≤ a.length, b.length ≤ 10^4

Clarifying questions1First question: Case-sensitive?Explore questions

Q1.Case-sensitive?
Assumed yes; equalsIgnoreCase for case-insensitive.

Quick interview revision

Understand

30-second explanation

Build the interview-ready mental model before comparing implementations.

If lengths differ, return false. Else compare each index; the first mismatch returns false. In Java use `.equals`, never `==` (which compares references).

Interview contextWhat is testedWhere candidates slipHow to stand outExplore interview context

What's being tested

Whether you know Java's == vs .equals distinction and compare correctly.

Where candidates slip

Comparing String references with ==.

How to stand out

Contrast Java (== is reference) with Python (== is value).

Choose

Compare the available approaches

Use constraints and trade-offs to select the route you can defend in an interview.

Learn & trace

Build each solution step by step

Move from the core idea to a visual trace, implementation, proof, and edge cases.

03.1

Approach 1 of 1 · Optimal

Character comparison

TimeO(n)SpaceO(1)

This is the standard, expected solution — lead with it.

Stuck? Progressive hintsReveal one at a time

1Hint 1Explore hint

What two conditions make strings equal?

2Hint 2Explore hint

Check length, then compare characters in order.

3Hint 3Explore hint

In Java, why is `a == b` the wrong way to compare strings?

A · Core idea

Two strings are equal when they have the same length and the same characters in order. Check length first, then compare character by character, returning false on the first mismatch.

Why length-check first. Different lengths can't be equal, and the check lets the loop index both strings safely with one bound.

Python. a == b does compare contents, so the language difference itself is worth calling out.

B · Visual dry run

Interactive dry run

Select a frame to connect the decision with its visual state.

a = "abc", b = "abd"

len

3 == 3

Stateok

Final resultfalse

C · Code with teaching comments

solution.javaJava

Character comparison — optimal

2 teaching notes

1
public boolean equal(String a, String b) {
2
    // Different lengths can't be equal.
3
    if (a.length() != b.length()) return false;
4
    for (int i = 0; i < a.length(); i++) {
5
        // First differing character.
6
        if (a.charAt(i) != b.charAt(i)) return false;
7
    }
8
    return true;
9
}

Yellow row = the algorithm's core logic. Amber `//` lines above it = what you'd say out loud.9 lines

D · Why it works and complexityO(n) timeO(1) spaceOpen the invariant and proofExplore the proof

The == vs .equals distinction in Java is one of the most common fresher bugs — call it out.

ComplexityO(n) time, O(1) space — length check is O(1), then up to n character comparisons with early exit.

E · Edge cases and pitfalls5a = "abc", b = "abc" — Identical → true.Explore edge cases

Edge cases handled

a = "abc", b = "abc"Identical → true.
a = "ab", b = "abc"Length differs → false.
a = "", b = ""Both empty → true.

Pitfalls

Using `==` on Strings in Java (compares references, not contents).
Skipping the length check.

Practice

Try the solution yourself

Edit the optimal implementation, test another input, and observe the result.

Try it — Java· run a complete program; method snippets need a driver

Avoid mistakes

Recognize the interview traps

Understand why common attempts fail before reviewing the corrected implementation.

Using == on Java Strings

== compares object identity; two distinct String objects with the same text are not ==. Use .equals for contents.

Explore the code comparison

Don't write this

if (a == b) return true;     // reference comparison

Write this instead

return a.equals(b);          // content comparison

Revise

Carry the pattern into the interview

Quick interview revision

Pattern to remember

Length + char comparison

Code pattern to memorize

len equal AND every a[i]==b[i]   (Java: use .equals)

Rules to remember

1Length must match.
2Compare chars in order.
3Java: use .equals, not ==.

Use this pattern when

Content comparison of strings

Choose another pattern when

Java == on strings — a reference-comparison bug

Interview takeaway

In Java, == on strings is a reference bug — use .equals.

Continue

Transfer the pattern to another problem

Use a direct variation or pattern sibling to prove the idea is reusable.

Transferable patternA core fresher pattern: Length + char comparison. Master it on small inputs and it transfers to harder problems.

Practice queueDirect variations and pattern siblings

Direct follow-ups

Valid Anagram
Equality of letter multisets, not order.